IS-IS creates flooding boundaries logical by breaking the network into routing domain levels i.e. A level2 routing domain and multiple level1 routing domains. The single Level 2 routing domain could be analogous to OSPF Area0 and multiple Level 1 routing domains are analogous to other OSPF Areas. Basically, IS-IS has two layer hierarchy: Level-2 (the backbone) Level-1 (the areas)     This two layer hierarchy is achieved by dividing the routers (IS’s) into three types called L-1, L-2, L-1-2 IS’s. Level1 Router is analogous to OSPF Internal non-backbone router (Totally Stubby). It contains a level1 LSDB only and is responsible for only routing … Continue reading IS-IS Levels & Relationships